This invention relates generally to magnetic recording of digital information, and more particularly to the recording of such information on magnetic disk files.
The magnetic disk file is a form of magnetic memory which is designed to permit relatively rapid random access of a large capacity of storage either for reading or writing of information. Physically, the file unit comprises a plurality of disks which are mounted upon a common shaft. Each disk has a magnetic recording layer (e.g. iron oxide) with recording tracks defined on either side of each disk. Magnetic heads are utilized to access the tracks as the disks are rotated.
Data is entered on the disk tracks in accessible sectors with identification headers provided for the sectors. As used herein "sector" or "record" is a set of data spaced from other data which may be randomly accessed for either reading or writing. Initially, when the lower density of data permitted greater tolerances in head placement for access, the headers for the data sectors could be placed on one disk surface and the data sectors on another disk surface. However, with the increasing density of data on the disks, the identification headers necessarily had to be placed on the same disk along with the data sectors.
Heretofore, each sector has been provided with its own identification header which precedes the sector on its record track. Not only does the header provide sector identification but the header also provides information as to the availability of the sector for record use and information regarding the chaining or linking of the sector with other sectors or related data storage. Further, many disk units permit variable length data sectors, and the sector headers must describe the storage capacity of its associated sector.
Since the header provides information as to the availability and capacity of the following data sector, along with chaining information, a gap must be provided after the header to allow sufficient time for the system control logic to make decisions relative to the availability of the sector and to initiate the control for reading or writing in the sector. Further, a sufficient length gap must be provided after each data sector to allow the system control logic to make decisions as to the validity of the previous information. Additionally, the gap lengths accommodate variations caused by asynchronous timing between the disk controller and the disk unit and inherent delay variations between disk drives due to circuit and cable delays.
Thus, the actual data information capacity of a disk is limited due to the necessity of providing headers and gaps for control logic decision time.